The relation of dimer size, double-stranded, circular DNA of the replicative form of the bacterial viruses phi X174, S13 and G4 to genetic recombination is being investigated in continuation of our previous work on this subject. A dimeric intermediate, the "figure-8", has been identified by electron microscopy that consists of one dimer-length and two monomer-length circular strands in the form of a covalently-closed duplex. The figure-8 is a configuration in which branch migration can occur. This process has been shown to be obligatory for recombination in the phi X-G4 system. The properties of the figure-8 are being studied by biochemical, genetic and physical methods including electron microscopy and analytical ultracentrifugation. These studies will permit its isolation and use in spheroplast tests of recombination; definition of its in vivo role in recombination; its conversion or other relation to circular and catenated dimers; and the study of the kinetics of branch migration. A bacterial plasmid system consisting of the Col El plasmid and a recently identified defective Col El plasmid is being developed for a similar study of recombination between circular genomes. A configuration of DNA derived from the figure-8 of phage G4 by treatment with the Eco R1 restriction enzyme has made available a DNA in which the kinetics of branch migration can be measured and factors evaluated that determine the rate and significance of the process in vivo. Physical studes are continuing of the kinetics of renaturation of covalently-closed, circular, duplex DNA.